Question

# 10.0 mL of a Cu2+ solution of unknown concentration was placed in a 250 mL Erlenmeyer...

10.0 mL of a Cu2+ solution of unknown concentration was placed in a 250 mL Erlenmeyer flask. An excess of KI solution was added. Indicator was added and the solution was diluted with H2O to a total volume of 75 mL. The solution was titrated with 0.20 M Na2S2O3. The equivalence point of the titration was reached when 14.45 mL of Na2S2O3 had been added. What is the molar concentration of Cu2+ in the unknown solution? 2Cu2+ + 4KI → 2CuI + I2 + 4K+ (rxn 1) I2 + 2Na2S2O3 → 2NaI + Na2S4O6 (rxn 2)

The given reactions are

2Cu2+ + 4KI → 2CuI + I2 + 4K+ (rxn 1)

I2 + 2Na2S2O3 → 2NaI + Na2S4O6 (rxn 2)

Overall reaction can be written as

2Cu2+ + 2Na2S2O3 + 4KI → 2CuI + 4K+ + 2NaI + Na2S4O6

As per stoichiometry of overall reaction, Cu2+ : Na2S2O3 react in molar ratio of 2:2 or 1:1.

Moles of Na2S2O3 used = 0.20M x 14.45 mL = 2.89 millimoles.

As the molar ratio is 1:1, so moles of Cu2+ = 2.89 millimoles.

Volume of Cu2+ taken = 10 mL.

Molarity = 2.89/10 = 0.289 M.